Radiation therapy is one of the most common therapeutic and palliative anti-cancer treatments. Its main limitation is due to the intrinsic radiation resistance of the tumor, limiting its efficacy (1). Because of the improvement of tumor imaging and medical physic researches new stereotaxic radiation therapy devices have been developed with a better targeting of the radiation into the tumor. Those stereotaxic accelerators are changing irradiation plans by increasing the dose within a limited number of fractions (1). If they demonstrated a strong efficacy in oligometastases and small solid tumors, these hypofractionated radiation therapy protocols have to be validated for most of the tumor type in function of their localisation and their radiation resistance.
Actually, the most common way to validate the radiation therapy efficacy is obtained through the visualisation of the tumor volume control or regression by CT-scan or by other non-invasive imaging techniques. Unfortunately, tumor volume response can be estimated within months after the end of the radiation therapy delaying any alternative treatment. Discovering biological markers allowing the discrimination between responding and refractory patients to the radiation therapy represents a major issue to improve anti-tumor treatment.
Biomarkers can be classified in three categories: omics from tumor biopsies, phenotypic imaging and secretory factors (2). Tumor markers by Omics are essentially obtained by genomics and proteomic assays. If they have the advantage to quest markers in a very large broad of molecular events, the need for tumor biopsies limits their studies to specific tumor localisation and the number of samples. Usually, those studies are dedicated to prognostic studies grading and assessing the treatment. Phenotypic imaging allows the evaluation of some physiologic change in the tumor such as hypoxia, cell proliferation index, necrosis or immune cell infiltration (3). Phenotypic imaging has the advantage to be non-invasive. However, the heterogeneity of the tumor response and the consistent quantification of the molecular biomarkers remain under investigation. Finally, secretory factors coming from blood, saliva and urine samples have the advantage to be easy obtained by any patient and can be provided before and all along the treatment (4). Those secretory factors can include pro-inflammatory cytokines, peptides LDL or circulating tumor cells. If some of them have been investigated, none of them have been validated as biomarker of the radiation therapy efficacy.
Sphingolipid ceramide also represent a potential and interesting secreted biomarker. Indeed, ceramide is a pro-apoptotic factor, generated rapidly into the outer layer of the cell membrane by the hydrolysosis of sphingomyelin by acidic or neutral sphingomyelinase (respectively ASM and NSM), but also in reticulum through a de novo synthesis pathway dependent of the ceramide synthase (5). Several studies demonstrate the involvement of ceramide in cell and tumor radiosensisitivity. Exogenous Ceramide treatment enhances radiation-induced LNCAP cell death and tumor regression (6). In the same manner, increasing endogenous ceramide through DL-PDMP and D-MAPP, respective inhibitors of glucosyl-ceramide synthase and ceramidase, enhances Jurkat radiosensitivity (7). Beside its involvement in tumor cell death, ceramide have been observed in endothelial cell apoptosis in response to high-dose radiation therapy which is modulating tumor regression. In fact, fibrosarcoma or melanoma tumor cells transplanted in mice, then irradiated, rapidly induced a massive endothelial cell apoptosis via ASM activation and ceramide generation participating to tumor regression (8). Invalidation of ASM blocks endothelial cell apoptosis and tumor regression induced by high dose radiation therapy.
Beside its intracellular form, secreted ceramide in the extracellular medium is also playing important biological roles in physiological and pathophysiological processes. High level of ceramide has been observed in plasma and serum from patients with several physiopathologies, including lung emphysema (9), Wilson disease (10), multiple organ failure (11). Plasma ceramide level is increased during lipid infusion in humans and rats, and in obese, insulin-resistant mice (12) which may correlated with insulin sensitivity, inflammation and atherosclerotic risk. Interestingly, ceramide and its enzyme ASM have also been quantified in serum from 11 patients with gross tumors from different origins after spatial fractionated grid radiation therapy (SFGRT) including a first irradiation at 15 Gy followed by 30 fractions of 2 Gy (13). Three days after treatment increase of secreted ceramide was quantified in the serum of 5 of the 7 patients responding to this specific radiation therapy protocol. However, the few number of patients and the diverse origin of the tumors diluted the strength of their results and do not allow to establish strong statistical evidence the correlation of ceramide and radiation therapy efficacy.